In comparison with projection-type Braun-tube displays, the projection-type color liquid crystal displays have the following excellent features, although they require a separate light source because their liquid crystal display element does not emit light: wide color-reproducible ranges are available, and they are portable because of their compactness and light weight; and it is not necessary to adjust convergence since they are not affected by earth magnetism. Therefore, further developments in this field are expected in the future.
There are two projection-type color-image displaying methods wherein liquid crystal display elements are used: a three-plate method wherein three sheets of liquid crystal display element corresponding to the three primary colors are used, and a single-plate method wherein only one sheet thereof is used. In the former three-plate method, an optical system for dividing white light into light beams of the three primary colors, that is, red, green and blue, and three sheets of liquid crystal display element for forming images by controlling the light beams of the respective colors are respectively provided, and the images of the respective colors are optically superimposed to form full-color images. In this arrangement of the three-plate method, it is possible to effectively utilize light emitted from the white-light source, while also obtaining high purity in colors. However, since the color-separation system for dividing colors and the color-composition system for superimposing colors are required as described above, the construction of the optical system tends to become complicated and the number of parts increases; therefore, this method is normally disadvantageous compared to the single-plate method in terms of low costs and miniaturization of the device.
In contrast, in the latter single-plate method, only one sheet of liquid crystal display element is used, and the projection optical system projects light onto the liquid crystal display element that is provided with a color filter having patterns of the three primary colors, for example, in the shape of mosaics or stripes. For example, Japanese Laid-Open Patent Application 230383/1984 (Tokukaishou 59-230383) has disclosed this method. The single-plate method, which uses only one liquid crystal display element and has a simpler construction in its optical system compared to that of the three-plate method, makes it possible to lower the costs and miniaturize the device.
However, in the case of the single-plate method, about two-thirds of incident light is absorbed or reflected by the color filter: only about one-third of incident light is utilized. In other words, the disadvantage of the single-plate method using the color filter is that the illumination of the screen is lowered to about one-third, compared to the three-plate method using a light source that has the same illumination.
In order to solve this problem, for example, Japanese Laid-Open Patent Application 60538/1992 (Tokukaihei 4-60538) has disclosed a color liquid crystal display of the single-plate method wherein, as illustrated in FIG. 14, white light from a white-light source 51 is divided into respective light beams of red, blue and green by the use of dichroic mirrors 54R, 54G and 54B that are disposed in the form of a sector and the utilization efficiency of light is thus improved.
In this display, the respective light beams, which have been divided by the dichroic mirrors 54R, 54G and 54B, are incident on a micro-lens array 55 that is disposed on the light-source side in a liquid crystal display element 57 with respectively different angles. The light beams, which have passed through the micro-lens array 55, are allocated and illuminated onto liquid crystal portions in accordance with the respective incident angles of the light beams. The liquid crystal portions are driven by signal electrodes to which color signals for the respective colors are individually applied. This display makes it possible to provide brighter images compared to displays wherein color filters are used.
However, the color liquid crystal display, which uses the dichroic mirrors 54R, 54G and 54B as the spectral means, has the following disadvantages:
First, the decline in the utilization of light causes a lowering in picture quality. More specifically, as illustrated in FIG. 15, the light beams, which have been converged by the micro-lens array 55 onto pixel apertures that are driven by the signal electrodes 56R, 56G and 56B in the liquid crystal display element 57, tend to diverge with great angles in an expanding manner after having passed through the liquid crystal display element 57; this causes a decline in the utilization of light and a resulting lowering in picture quality.
In contrast, it is possible to project images of good quality onto a screen 60 by employing a lens with a large diameter as a projection lens 59 shown in FIG. 14. However, since the lens is normally an expensive member, the production costs increase, which is the second problem.
Moreover, the inventors of the present invention have pointed out the third problem in the display that is disclosed in the above-mentioned patent publication, that is, the decline in the purity of the three primary colors that might adversely affect the quality of images. This disadvantage is caused by the fact that no specific consideration is given on the arrangement of the dichroic mirrors 54R, 54G and 54B, that is, on the order of dividing colors, as well as the fact that multiple reflection occurs between these dichroic mirrors 54R, 54G and 54B disposed in the form of sector and mixed colors thus tend to be caused.
Referring to FIG. 16(a), the following description will discuss this problem in detail. Here, the figure exemplifies a case where the dichroic mirrors 54B, 54G and 54R, which respectively reflect light beams of blue, green, and red wavelength ranges, are disposed in the form of sector in this order from the white-light source, with their angles shifted by .theta. respectively. Here, .alpha. represents an angle at which white light is incident on the dichroic mirror 54B.
The white light, which is directed to the dichroic mirrors 54B, 54G and 54R, is divided into the following three light beams:
(1) a blue light beam that has been reflected by the dichroic mirror 54B; PA1 (2) a green light beam that passed through the dichroic mirror 54B, was reflected by the dichroic mirror 54G, and again has passed through the dichroic mirror 54B; and PA1 (3) a red light beam that passed through the dichroic mirrors 54B and 54G, was reflected by the dichroic mirror 54R, and again has passed through the dichroic mirrors 54B and 54G. In this case, the green light beam is incident on the liquid crystal display element 57 with a tilted angle of 2.theta. in its travelling direction with respect to the blue light beam, and the red light beam is also incident thereon with a tilted angle of 2.theta. in its travelling direction with respect to the green light beam. PA1 (1) a white-light source for emitting a white-light beam; PA1 (2) a light-beam divider for dividing the white-light beam into light beams consisting of a plurality of color rays having respectively different wavelength ranges; PA1 (3) a liquid crystal display element including a face whereon pixels corresponding to the respective color rays are regularly disposed, the liquid crystal display element being arranged so that the incident color rays are modulated and transmitted through the pixels; PA1 (4) a first micro-lens array for converging the color rays of the light beams onto corresponding pixel apertures in the liquid crystal display element, each color ray being allocated with respect to each wavelength range; and PA1 (5) a second micro-lens array for deflecting the light beams so that their respective principal rays are aligned in parallel with one another. PA1 (1) a white-light source for emitting a white-light beam; PA1 (2) a light-beam divider for extracting a plurality of color rays having respectively different wavelength ranges from the white-light beam in the order of their greater wavelength ranges starting from the longest wavelength side; PA1 (3) a liquid crystal display element on which the color rays are incident and through which the color rays are modulated; PA1 (4) a micro-lens array for converging the color rays onto corresponding pixel apertures in the liquid crystal display element, each color ray being allocated with respect to each wavelength range; and PA1 (5) a projector for projecting the color rays that have been modulated by the liquid crystal display element. PA1 (1) the first dichroic mirror for reflecting the red ray; PA1 (2) the second dichroic mirror for reflecting the yellow ray; and PA1 (3) the third dichroic mirror for reflecting the blue ray. PA1 (1) a polarizing plate that is installed on the light-incident side in the liquid crystal display element; and PA1 (2) a polarization-axis rotator for rotating the polarization axis of either p-state polarization or s-state polarization to a direction in which the polarization axis is coincident with the transmitting axis of the polarizing plate, the polarization-axis rotator being installed between the light-beam divider and the polarizing plate. PA1 (1) a white-light source for emitting a white-light beam; PA1 (2) a light-beam divider for dividing the white-light beam into light beams consisting of color rays having respectively different wavelength ranges; PA1 (3) a liquid crystal display element on which the color rays are incident and through which the color rays are modulated and transmitted; PA1 (4) a screen; PA1 (5) a projection lens for projecting onto the screen the color rays that have been modulated by the liquid crystal display element; and PA1 (6) a wavelength selector having selection areas that transmit only rays having wavelength ranges corresponding to the respective color rays, the wavelength selector being installed on the entrance pupil of the projection lens.
Here, in actual process, stray light is exerted due to unnecessary reflections, in addition to the above-mentioned light beams. The following description will discuss the causes of stray light in detail.
The dichroic mirrors 54B, 54G and 54R, which are manufactured through a well-known multi-layer thin-film coating technique, have respectively different spectral characteristics depending on the incident angles of light beams. For this reason, each angle at which each light beam is incident is individually determined at the time of designing the mirror in order to obtain desired spectral characteristics (hereinafter, this angle is referred to as the designed incident angle). Therefore, if a light beam is incident at an angle different from the designed incident angle, the desired spectral characteristics are not obtainable, and as the gap between the designed incident angle and the actual incident angle increases, the actual spectral characteristics further diverge from the desired spectral characteristics.
FIG. 16(b) shows the spectral characteristics of the dichroic mirror 54B having the designed incident angle of 45.degree. (which reflects the light beam of a blue wavelength range and transmits the other light beams of the other wavelength ranges) and actual spectral characteristics that the dichroic mirror 54B exhibits when a light beam (natural light) is incident thereon at an angle of 20.degree. that is different from the designed incident angle. Here, in this figure, the spectral characteristics of the 45.degree.-incident angle are indicated by a solid line, and the spectral characteristics of the 20.degree.-incident angle are indicated by a broken line. As clearly shown by the figure, in the case of an incident light beam having an angle smaller than the designed incident angle, a rise in transmittance, which was located in the vicinity of 500 nm, is shifted to the long-wavelength side. Further, ripples (swells like sinusoidal waves in the transmittance curve) appear in the characteristic curve. Furthermore, a step-like portion appears in the vicinity of 50% in transmittance within the portion of the rise. This step-like portion is caused due to a discrepancy in the spectral characteristics with respect to the s-state polarization and p-state polarization in the case of the incidence of natural light, and gives adverse effects on the characteristics in the same manner as the ripples.
For example, when non-polarized natural light is illuminated, the green light beam, which has been reflected by the dichroic mirror 54G, is again incident on the dichroic mirror 54B at an angle that is smaller by 2.theta. than the designed incident angle .alpha. of the dichroic mirror 54B. Therefore, the spectral characteristics of the dichroic mirror 54B are changed, thereby making the reflection range shift toward the long-wavelength side, as well as causing increased ripples. Thus, a portion of the green light, which is supposed to pass through the dichroic mirror 54B, is reflected by the dichroic mirror 54B.
In this manner, as illustrated in FIG. 17(a), stray light M is exerted, although it is a small portion, and when this stray light M again reaches the dichroic mirror 54G, most of the stray light M is reflected by the dichroic mirror 54G. The stray light M, reflected by the dichroic mirror 54G, is again incident on the dichroic mirror 54B. At this time, its incident angle is smaller than .alpha. by 4.theta., which is the same angle as the light beam of red that is reflected by the dichroic mirror 54R. The travelling direction of the stray light M having passed through the dichroic mirror 54B also makes the same angle as the red light beam does after having passed through the dichroic mirror 54B, and has an angle difference of 4.theta. with respect to the light beam of blue that the dichroic mirror 54B has first reflected. This means that the stray light M of green is slightly contained in the pixels for modulating the red light beam in the liquid crystal display element 57.
Similarly, the red light beam, which has been reflected by the dichroic mirror 54R, is incident on the dichroic mirrors 54G and 54B at angles that are smaller than the designed values by 2.theta. and 4.theta. respectively. For this reason, each of the dichroic mirrors 54G and 54B has a shift in its rise in the spectral characteristics in the same manner, thereby causing a portion of the red light beam to be reflected by the dichroic mirrors 54G and 54B. Stray light N, which is caused in this case, is light that was reflected by the dichroic mirror 54R, and passed through the dichroic mirrors 54G and 54B, or light that was reflected by the dichroic mirror 54G, and passed through the dichroic mirror 54B. This stray light N, which has an angle difference of 2.theta. with respect to the red light beam, is incident on the liquid crystal display element 57 at an angle that is different from any angles of the blue, green and red light beams.
There are still other stray lights that are caused by further reflections of light; however, these stray lights hardly give adverse effects on the purity in colors. The reason for this is that the light intensity is decreased as the reflections are repeated, and that the incident angles onto the liquid crystal display element are increased with respect to the optical axis as the reflections are repeated so that they exceed the effective diameter of the projection lens that is regulated by the F-value thereof.
As illustrated in FIG. 17(b), the stray lights M and N cause mixed colors when the micro-lens array 55 allocates the respective light beams onto the pixels that are driven by the signal electrodes 56B, 56G and 56R in the liquid crystal display element 57. Additionally, the signal electrodes 56B, 56G and 56R are associated with the respective colors, blue, green, and red, and these three electrodes form a unit to which a predetermined one of micro-lenses constituting the micro-lens array allocates the light beams.
In this arrangement, the stray light M (green) is incident on the liquid crystal display element 57 at the same angle as the red light beam that has been reflected by the dichroic mirror 54R, and is thus incident on the signal electrode 56R together with the red light beam. Moreover, the stray light N (red) tends to be incident on a signal electrode 56B', shown in FIG. 17(b), that is a signal electrode that is associated with another micro-lens. Consequently, the purity in the three primary colors deteriorates due to stray lights.